Electric discharge machining is a machining method in which an electrode and a workpiece oppose each other across a machining gap, and a voltage is applied between the electrode and the workpiece to generate acontinuous electric discharge between the two so as to perform machining. The energy to create the electric discharges is supplied as a pulse current from a power supply device. This energy is applied to the workpiece as electric discharge energy to machine the workpiece. When an electric discharge is generated between the electrode and the workpiece, there is a sudden rise in temperature due to electric discharge energy at the electric discharge generation point (the electric discharge point) of the workpiece, whereby the workpiece material locally melts and vaporizes. At the same time, a sudden rise in pressure occurs at the electric discharge point, and the melted, vaporized material is blown away, forming a crater-shaped hole in the workpiece. Electric discharge machining is carried out by generating a continuous electric discharge in the machining gap formed between the electrode and the workpiece while a dielectric fluid (typically a high resistively machining fluid)--for example mineral oil or water having extremely high relative resistance (deionized water) is interposed therein. Therefore, after an electric discharge is generated, the electric discharge point is cooled by the machining fluid, while at the same three the machining waste produced is removed from the vicinity of the electric discharge point.
It is known that electric discharge energy is consumed not only for the removal of the workpiece material, but also for removal of material from the machining electrode side, and results in damage to the electrode. This is usually referred to as electrode wear. The phenomenon of electrode wear is difficult to avoid in electric discharge machining. The wear rate of electrode material relative to the amount of material removed from the workpiece varies greatly depending upon the conditions of the electric energy supplied to the machining gap--which include for example the electric discharge time, the electric discharge current, the electrode and the workpiece polarity, and the electrode and the workpiece material. Therefore electric discharge machine operators typically select and set appropriate machining conditions for different workpiece machining stages, such as rough-cuts, second-cut (or mid-cuts) and finish cuts.
In a vertical (or die sinking) electric discharge machine, rough cut machining for roughing out a shape, to mid-cut machining for machining to a machined surface roughness of approximately 5 .mu.m Rmax, is typically performed by supplying pulse current having a peak current value in the range of approximately 10 to several 100 amperes over an electric discharge time in the range of approximately 10 to several 1000 .mu.sec. When machining under such conditions, the working electrode is usually connected to the positive pole of the electric discharge power supply circuit, and the workpiece is usually connected to the negative pole thereof. With such machining conditions, i.e., using such discharge times and peak current values, machining can be carried out with an electrode wear of less than a few percent by setting the electric discharge time and peak current values to be in a specified ratio. For conventional finish machining, that is for a finish surface roughness of less than 5 .mu.m Rmax, a peak current energy of less that a few amperes at a short electric discharge time in the range of from about 10 .mu.sec to 0.5 .mu.sec is used. When operating in this regime, the working electrode is usually connected to the negative pole of the electric discharge power supply circuit, and the workpiece to the positive pole. As compared to connecting the working electrode to the positive pole of the electric discharge power supply circuit, using a negative polarity working electrode results in an approximately 30% decrease in electrode wear. In other words, machining with a positively poled working electrode and discharge times in the range of from about 10 to 0.5 .mu.sec results in a higher wear rate electrode as compared to a negatively polarity working electrode. Further, using a negatively poled working electrode results in stable electric discharge machining and a fine surface finish at several times the machining speed as would be obtained using a positively poled working electrode.
As it relates to machining speed, in contrast to vertical electric discharge machine, the machining conditions used in a wire cut electric discharge machine typically comprise an electric discharge time in the range of from about 100 nsec-10.mu.sec and an electric discharge current peak value in the range of approximately 1600 A to 1 A. With regard to polarity, machining is usually performed with the wire electrode connected as the negative pole. This is because with the reverse polarity, the electrode would tend to wear excessively; not only would machining not progress well, but the wire electrode would also be likely to open (break). Therefore, in a wire cut electric discharge machine, except for cases in which an alternating current pulse is supplied, the wire electrode is almost always held at a negative polarity relative to the workpiece. The connection of the power supply cable is physically fixed such that the wire electrode is connected to the negative pole of the power supply, and the workpiece to the positive pole.
During wire cut electric discharge machining, a wire made of brass, copper, tungsten or other material, wound around a supply reel and having a diameter of from about 0.03 mm to 0.35 mm, is fed from the spool and guided along a wire travel path by means of a wire guiding device in which wire guides are positioned on either side of the workpiece. The wire is continuously fed while applying a specified tension to it as it travels trough the machining zone. An electric discharge is generated between the tensioned portion of the wire electrode and the workpiece. A typical wire cut electric discharge manufacturing process begins with a rough cut, also referred to as a "first cut," in which the workpiece and the wire electrode are moved relative to one another under control of a numerical control (NC) device while the electric discharge is generated, thus forming a desired profile shape in the workpiece. Typically the next step(s) is to further refine the machined surface roughness while increasing the accuracy of the machined shape. During this step(s), the profile path over which the wire electrode is guided is shifted by a specified amount toward the product side, and the surface and shape of the workpiece which will become the product of the process are finished under lower electrical energy conditions. This step is referred to as the second cut or third cut (or more generally mid-cut), depending on the number of passes.
In recent years there has been a tendency to increase the final product machining roughness requirement, from less than about 2 .mu.m Rmax to less than about 1 .mu.m Rmax. However, the electric discharge energy used to achieve a surface roughness of less than 2 .mu.m Rmax is extremely small, and requires that the machining gap be quite narrow. For this reason, the greater the demand for finer machined surface roughness, the smaller, proportionally, must be the electric discharge energy, making machining difficult. Under such finish machining requirements the machining gap must be maintained at only a few microns. Given such spacing, the static electricity forces at work between the wire electrode and the workpiece increases, such that a force works to tend to pull the wire electrode toward the workpiece. In machining regions in which the electric discharge energy is of a certain strength, the electric discharge pressure generated by electric discharge works as a reaction force, so the effects of the static electric force are not conspicuously manifested. But, when electric discharge energy is small, the electric discharge reaction force is also small, and a vibration phenomenon may be created in the wire electrode when affected by the static electric forces. This makes stable machining very difficult. Furthermore, the effects of wire electrode vibration may be manifest on the workpiece surface in the form of line-shaped marks, such that the desired surface roughness cannot be obtained.